Epoxy resin preparations comprised of aliphatic, cycloaliphatic, or aromatic macromolecules having reactive oxirane groups are well known in the art. They are known to form highly crosslinked thermosets when treated with curing agents. Innumerable combinations of epoxy resins and curing agents have been prepared and found useful in a wide range of applications such as laminates, adhesives, coatings, and structural components. Most have required the use of organic solvents so that desirable curing agent/resin combinations could be placed into solution.
Epoxy resin/curing agent combinations can be formulated into one-pack or two-pack compositions. In two-pack formulations, the components are separated and will cure, generally at room temperature, when combined. Metering the proper amounts of the components with the proper amount of mixing and other process conditions is important. Attaining the proper formulations with precision and accuracy can be difficult in certain circumstances. These systems can also have relatively short pot lives. One-pack systems can be used to overcome such difficulties. These systems employ a latent curing agent that is premixed with the epoxy resin. Curing occurs upon heating the mixture. Accelerators may also be used to reduce the severity of curing conditions or hasten the curing process.
Environmental and safety concerns have provided an impetus for the continued development of epoxy compositions that do not require the use of volatile solvents. Increasingly, manufacturers are attempting to produce completely aqueous systems. Unfortunately, many require a multitude of components which are necessary to obtain the desired properties of the applied or cured epoxy formulation. One-pack aqueous systems thus pose a significant challenge to the formulator.
Dicyandiamide (hereafter "DICY") is a particularly desirable latent epoxy curing agent. Its latency and ability to facilitate the formation of relatively impervious, dense cross linked polymers would be beneficial in one-pack systems. DICY has the following structure: ##STR1##
Indeed, the use of DICY as a hardener for epoxy systems is widespread in many industrial processes including epoxy resin prepregs, structural adhesives, saturants for paper and polyester cloth, as a sizing for glass fibers, and in industrial coatings. Chemically stable varnishes and partially reacted coated (B-staged) articles which can be handled and maintained ready for subsequent cure can also be fabricated from epoxy resin-DICY combinations. Articles that are prepared in this way typically have high thermal tolerances when compared to other epoxy systems. Thus, DICY cured epoxy resin systems bring extended pot life, high crosslink density, excellent chemical resistance, low color, "B-stagability," and high temperature to the cured article. In most epoxy resin systems, the DICY itself comprises only between about 2 and 10 parts per hundred. Thus, a solvent or mixture of solvents are necessary to produce a homologous epoxy-curing agent system. This poses a problem if one wishes to use DICY as the curing agent since it is not readily soluble in either epoxy resins or most solvents. For example, at room temperature, only 0.05% wt DICY is soluble in the liquid diglycidyl ether of Bisphenol A sold commercially as EPON.RTM. Resin 828 (a product of Shell Chemical Company). At 150.degree. C. the solubility is only increased to about 0.6 g per 100 gms of resin. Formulators are often left only with the option to employ exotic and sometimes toxic solvents such as dimethyl formamide, propylene glycol monomethyl ether, and n-methyl pyrrolidione.
DICY is somewhat more soluble in water (4.13 g/100 g H.sub.2 O at 25.degree. C.) than it is in most common solvents. Thus, some have proposed waterborne solutions where DICY is the curing agent. This is especially desirable in light of the environmental and safety factors noted above. Unfortunately, preparing adequate aqueous solutions in which DICY is a component has still posed significant challenges. Laborious and difficult processes have been required to get the DICY into solution. Even where this has been done, the stability of the systems has not proven reliable.
The crystallinity of the DICY molecule has much to do with its difficult behavior in epoxy resin dispersions. For example, one commercial practice requires about 10% wt of DICY to be dissolved in water at elevated temperatures. This solution is then added to an epoxy dispersion. However, the DICY/water solution must be kept hot and must be added to an epoxy dispersion before the DICY begins to crystallize out of solution. In practice, this is rarely achieved with good results. The crystals often agglomerate and settle out of solution leaving final dispersions containing large DICY crystals. Even where this agglomeration is not extensive there is almost always some DICY crystalline matter present. This can cause settling and instability of varnishes and dark spots on cured articles. Additionally, because the DICY must be added as a dilute solution, the resin/curing agent solids are compromised in the final product by decreasing the maximum resin pickup of the article.
U.S. Pat. No. 4,421,877 proposes an aqueous emulsion of epoxy resins, flame retardant phenol, monomethylol dicyanciamide or dicyandiamide curing agent, and particular nonionic alkylaryl polyether alcohol surfactants. Preparing the emulsion involves the addition of two separate solutions; an epoxy resin/phenol and surfactant solution, and a curing agent in water solution is formed. The two solutions, prepared separately, are combined. Stability of the ultimate emulsion was not demonstrated beyond two days.
The art would be benefitted by a method of producing a stable waterborne one-pack DICY curing epoxy resin system.